From 0533f6d1a6fb12f428f2a44d9b4877ece37aa98e Mon Sep 17 00:00:00 2001
From: Tim Ledbetter <tim.ledbetter@ladybird.org>
Date: Thu, 7 May 2026 22:16:39 +0100
Subject: [PATCH] AK+LibWeb: Add a `fast_as<T>()` method for quickly casting
 mixin types

`as_if<T>()` previously had a fast path that used `fast_is<T>()` and a
static cast. Mixin types couldn't use this fast path, since a static
cast from a sibling base is not valid, so would fall back to using a
slow dynamic cast instead.

This change adds a `fast_as<T>()` mechanism for mixin types to opt in
to and teaches `as_if<T>()` to use it. This means `as<T>()` and
`as_if<T>()` can be used with mixin types without incurring the
overhead of a dynamic cast
---
 AK/TypeCasts.h              | 4 +++-
 Libraries/LibWeb/DOM/Node.h | 5 +++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/AK/TypeCasts.h b/AK/TypeCasts.h
index d9c924bca0..7141d3b8a3 100644
--- a/AK/TypeCasts.h
+++ b/AK/TypeCasts.h
@@ -38,7 +38,9 @@ ALWAYS_INLINE bool is(NonnullRefPtr<InputType> const& input)
 template<typename OutputType, typename InputType>
 ALWAYS_INLINE CopyConst<InputType, OutputType>* as_if(InputType& input)
 {
-    if constexpr (requires { input.template fast_is<RemoveCVReference<OutputType>>(); static_cast<CopyConst<InputType, OutputType>*>(&input); }) {
+    if constexpr (requires { input.template fast_as<RemoveCVReference<OutputType>>(); }) {
+        return input.template fast_as<RemoveCVReference<OutputType>>();
+    } else if constexpr (requires { input.template fast_is<RemoveCVReference<OutputType>>(); static_cast<CopyConst<InputType, OutputType>*>(&input); }) {
         if (!is<OutputType>(input))
             return nullptr;
         return static_cast<CopyConst<InputType, OutputType>*>(&input);
diff --git a/Libraries/LibWeb/DOM/Node.h b/Libraries/LibWeb/DOM/Node.h
index 7bc07851fa..f6573fa005 100644
--- a/Libraries/LibWeb/DOM/Node.h
+++ b/Libraries/LibWeb/DOM/Node.h
@@ -379,6 +379,11 @@ public:
     template<typename T>
     bool fast_is() const = delete;
 
+    template<typename T>
+    T* fast_as() = delete;
+    template<typename T>
+    T const* fast_as() const = delete;
+
     WebIDL::ExceptionOr<void> ensure_pre_insertion_validity(JS::Realm&, GC::Ref<Node> node, GC::Ptr<Node> child) const;
 
     bool is_host_including_inclusive_ancestor_of(Node const&) const;